FIG. 5 is a half sectional view of one example of a conventional loudspeaker. This loudspeaker is substantially axially symmetric with respect to a dashed line C-C, namely a center line. In FIG. 5, a closed loop of a direct-current magnetic circuit is formed of disk-like plate 1 made of silver, disk-like magnet 2 one-size smaller than plate 1, iron yoke 3, and a narrow clearance (magnetic gap G) between plate 1 and yoke 3. Voice coil 4 is disposed in magnetic gap G, and can move vertically on the drawing. Voice coil 4 is coupled to the inner peripheral end of diaphragm 6 via cylindrical voice coil body 5. Diaphragm 6 vibrates to produce sound. The outer peripheral end of diaphragm 6 is coupled to frame 8 via flexible edge 7. The back surface of diaphragm 6 is coupled to frame 8 via suspension holder 9 and flexible edge 10.
The loudspeaker of FIG. 5, among conventional loudspeakers, has relatively high sound quality. In this loudspeaker, edge 7 and edge 10 project in opposite directions to each other, so that a symmetric characteristic of the vertical amplitude of diaphragm 6 is improved, and this symmetric characteristic reduces sound distortion in the loudspeaker. Since the sound distortion can be reduced, large current can be applied to voice coil 4, and louder sound can be output.
An example of the conventional art document information related to the invention of this application is Japanese Patent Unexamined Publication No. 2004-7332.
When a large current is applied to voice coil 4 in order to output louder sound, however, sound distortion due to the structure of the closed loop magnetic circuit becomes a problem. This sound distortion has not caused a problem before. This phenomenon is described hereinafter.
FIG. 6 is an enlarged side view of the proximity of magnetic gap G. Magnetic gap G refers to a part where plate 1 is closest to yoke 3. In magnetic gap G, magnetic flux F flows from plate 1 toward yoke 3. The vertical center position of the magnetic flux F part is generally set as the center position of the vertical movement of voice coil 4, and voice coil 4 is movable vertically from the center position of the vertical movement. When a large current is applied to voice coil 4 in order to output louder sound, however, the lower end of voice coil 4 moves with a large amplitude downwardly below the bonded surface between magnet 2 and yoke 3 of FIG. 6. On the bonded surface between magnet 2 and yoke 3, however, leakage magnetic flux L returning from a right midway part of yoke 3 to the lower surface of magnet 2 exists. Leakage magnetic flux L flows in the opposite direction to magnetic flux F. When voice coil 4 comes down and enters leakage magnetic flux L, voice coil 4 receives an upward force in the opposite direction to the force from magnetic flux F. When the upward force enlarges the amplitude of voice coil 4, the symmetric characteristic of the amplitude degrades to cause the sound distortion, disadvantageously.